Conveyor belts and transfer chutes are equipment used for the transport of various materials, in casu, for the transport of ores.
The conveyor belt consists of a device basically formed of an endless belt which is extended between two drive drums (driving and return) and an internal structure constructed by laminated profiles and juxtaposed rollers, over which the belt slides enabling the movement of the ore positioned on the belt.
The transfer chute consists of a device commonly applied for the transfer of material between conveyor belts that operate in different directions. This basically consists of a funnel, formed by associated steel plates and wear material, assembled to intermediate the transfer of material.
Conveyor belts and transfer chutes are used specifically in this case to carry the ore that arrives at the boarding terminal by rail. After homogenization on stacks and ore recovery, this goes through conveyor belts and transfer chutes to the ships for carrying out the transport to the final destination. However, not rarely, the ore transported by such equipment has a considerable moisture content, which is a detrimental characteristic to maritime transport.
The moisture content of the ore carries serious drawbacks to its shipping. The first one is related to the cost of freight as each unit of water transported represents additional costs, such as penalties to the supplier as the sold ore is assessed on a dry basis. In addition, it decreases ore transport capacity, causing significant losses.
Nevertheless, it is known that the maritime transport of excessively wet bulk solid cargo may imply a risk of tipping and cargo ship sinking, due to a phenomenon known as “granule load liquefaction,” which occurs when moist ore is submitted to boat balance, engine vibration of the vessel and the successive impacts from the sea to the ship hull. When the ore is with a moisture percentage above a value corresponding to a Flow Moisture Point (FMP) and is subjected to such vibration conditions, it can liquefy. When the material liquefies, this viscous mixture can move improperly in the holds of the vessel to the bottom and/or walls, unbalancing and eventually leading the ship to sink due to inertial forces acting on the ore cargo/ship.
So to prevent that ores with high moisture content are transported, government technical standards created the TML (Transportable Moisture Limit), which is the maximum amount of moisture that the ore must contain in order to be fit for transport vessels. In practice, the value adopted for the TML is equivalent to 90% of the FMP.
Thus, the use of equipment and processes to ensure compliance with the TML requirements for moisture of the ore prior to shipment of the material is critical.
In the state of the art, can be used various types of drying equipment such as, rotary kilns. These devices usually have high installation costs, operation and maintenance mainly due to high energy consumption. In addition, it is necessary that this drying equipment is installed in series with transport equipment (conveyor belts, chutes, among others). So changes were needed in the transportation lines for the installation of this kind of drying equipment, resulting in layout changes—process flowchart—due to the need for more room. Nevertheless, it would take longer breaks in the transportation lines for installation, which significantly raise costs and undermine substantially the flow of production, also implying in need of large areas for storage yards.
Thus, the use of state of the art drying equipment to reduce ores moisture, implies high costs on the acquisition, installation, operation and maintenance of equipment.
The state of the art also includes devices for drying, or even dehydration of foods and other materials. One of this equipment is revealed in the document U.S. Pat. No. 2,395,933.
The equipment disclosed in U.S. Pat. No. 2,395,933 is set to generate a hot air flow directed into the material being transported by a conveyor belt. Such equipment comprises a conveyor belt, a fan, a heater and specific meters.
The conveyor belt is set to perform transportation of the material along the machine, and the environment is isolated to allow the air blown by the fan remains inside the machine.
The fan is set to blow air into the equipment, and it is installed in series with the heater. The fan blades are set to direct air blown by the fan into the material being transported. These can be oriented in the direction that is most convenient to the process, such that all material present on the belt is dehydrated. Also temperature measurements are performed, air moisture and material as well as other variables involved.
Although the equipment disclosed in document U.S. Pat. No. 2,395,933 perform a reduction of moisture products transported on a belt, it is not suitable for moisture reduction of ores. The above document equipment does not previously perform moisture removal from the air used by the process, so that it shows a lower efficiency during the drying of the material.
Other equipment comprised in the state of the art is disclosed in document U.S. Pat. No. 2,415,738. This document presents equipment for partial dehydration of cellulose containing products, food products and other industrial materials. Such equipment consists of a conveyor belt set in isolated environment within injection of hot gases.
The equipment comprises a conveyor belt, exhaust ducts, gas compartment, pumps, and monitoring system. The environment in which the conveyor belt is located is isolated in such a way that the gases remain inside, and the pumps are set to inject hot gas in that isolated environment.
The intake duct is responsible for the admittance of gases, while the exhaust duct is set to remove the excess gases of the internal environment. The exhaust duct also allows the reuse of gases removed, returning them to the drying compartment.
Measurements of the variables involved in the process such as temperature and flow rate of the gases are performed. The variable monitoring system allows changing the machine settings, ensuring its better functioning.
Similarly to patent document U.S. Pat. No. 2,395,933, the document U.S. Pat. No. 2,415,738 discloses a device that carries out a reduction in moisture contained in industrial materials carried in a belt, however, this equipment is not suitable for moisture reduction of ores. The above document equipment does not previously perform moisture removal from the air used by the process, so that it shows lower efficiency and high energy consumption during the drying of the material.
Owing to the equipment described in these documents, there is not, in the state of the art, a process or equipment applied for ores moisture reduction, to percentages below the TML, which has low installation, operation and maintenance cost.